^ even if it shows up in BIOS, that doesn't mean that your system will actually use all that ram in a 64 bit OS, that is just the reading of actual ram installed, not what will be acceptable for the system.
Start > Control Panel > System
That page will show you your installed ram and what is actually being used.
Mine says 4.00gb (3.00gb usable)
Just give us your Motherboard specs to clarify, also your CPU as it may require FSB Parity.